1. Field of the Invention
The present invention relates to a ferrule for an optical connector attached to the end of an optical fiber, and more particularly to a ferrule for an optical connector formed of a resin. The present invention also relates to a metal article having a through-hole.
2. Description of the Related Art
In a communication network using optical fibers, optical connectors are used to connect optical fibers together. The optical connector requires a ferrule for concentrically butting ends of optical fibers together.
FIG. 6 illustrates a sectional view of a conventional ferrule. In FIG. 6, numeral 1 denotes a ferrule body, numeral 2 represents a through-hole formed of (i) a small diameter portion 3, (ii) a large diameter portion 4, and (iii) a tapered portion 5 expanding toward the large diameter portion 4 from the small diameter portion 3, and numeral 6 denotes a collar portion. The ferrule body 1 is formed of a resin and molded by injection molding using a metallic mold. The small-diameter portion 3 and the large-diameter portion 4 are formed so that center axes thereof are axially aligned. In this ferrule, an optical fiber 7 is inserted, where a length of core wire 8 is inserted into the large diameter portion 4, a length of elementary wire 9 obtained by stripping the hull of the core wire is inserted into the small diameter portion 3, and the end of the elementary wire 9 is positioned at the end of the small diameter portion 3.
A conventional ferrule is formed of an epoxy resin, etc., having a higher thermal coefficient of expansion than that of the an optical fiber. This results in slippage of a butting position of glass portions of the optical fibers and increases transmission loss of optical communication.
In order to improve this problem, a double structure concentrically covering a plastic pipe body with a metallic hollow-body cylinder has been proposed as in Japanese Unexamined Patent Publication No. 58-27112.
According to this structure, it is asserted that deformation and changes in size such as contraction of the plastic material can be prevented and abrasion of the cylindrical collar due to mating and unmating operations of the plug can be well suppressed, thereby offering the advantage of sturdy connections.
Another proposed ferrule structure is formed of two concentric metallic pipes having different outer diameters in the front portion of the ferrule for connecting to a sleeve, etc., and in the back portion of the ferrule and is unitized by a resin with an optical fiber inserted therein. This structure is disclosed in Japanese Unexamined Patent Publication No. 61-137109, for example.
It is asserted that this structure has the advantages of: (i) increased bending strength because the metallic pipes having different outer diameters reinforce the mechanical strength of the resin; (ii) smaller change in size with changes in temperature (i.e., improved thermal coefficient); and (iii) preventing bending of the core wire of the optical fiber during molding of the resin.
Still another proposed structure includes a resin-made connector in which a portion of a metallic ferrule (with an end of an optical fiber cord inserted therein) is crimped and the ferrule is covered by a resin to obtain a unitarily molded item. This structure is disclosed in Japanese Unexamined Patent Publication No. 2-82206. Since the ferrule is crimped relative to the optical fiber and unitarily molded by the resin, it is asserted that the optical fiber can be prevented from separating from the connector and heat and pressure is not directly transmitted to the optical fiber (due to the metallic ferrule) during molding.
However, in the ferrule as disclosed in Japanese Unexamined Patent Publication No. 58-27112, since the elementary wire touches the plastic pipe directly and the plastic has a higher thermal coefficient of expansion, a butting position of the elementary wires of the optical fibers changes and an increase in transmission loss of optical signals results.
The ferrule according to Japanese Unexamined Patent Publication No. 61-137109 requires that the metallic pipes having different outer diameters are arranged concentrically and, therefore, positioning during the manufacturing process is difficult. Further, the connecting position of the optical fibers in the butting position of the ferrule is unstable and adversely affects the transmission loss characteristics of the device.
The ferrule disclosed in Japanese Unexamined Patent Publication No. 2-82206 is formed of a metal; in particular, brass having a large thermal coefficient of expansion is used. Disadvantageously, only crimping is employed to fix the ferrule and prevent the optical fiber from separating from the connector. Therefore, no consideration is given to changes in butting position of the elementary wires of the optical fibers, specifically due to changes in temperature.
The present invention can solve the aforementioned problems associated with prior art devices and provides a resin ferrule for an optical connector which reduces thermal expansion and contraction resulting from changes in ambient temperature. The present invention further provides a method for manufacturing the resin ferrule. The present invention further provides a novel metal article having a through-hole used for producing the resin ferrule or other devices.
A ferrule for an optical connector according to the present invention comprises a ferrule body formed of a resin with a through-hole formed therein so that an optical fiber can be inserted therein and separated therefrom, the through-hole including a large-diameter portion and a small-diameter portion which are axially aligned; and a metallic pipe unitarily arranged on the inner wall of the through-hole, the metallic pipe being formed of an alloy of iron-nickel or an alloy of iron-nickel-cobalt.
In the ferrule for an optical connector according to the present invention, a thermosetting resin or a thermoplastic resin is preferably used to form the ferrule body. Examples of thermosetting resins include epoxy resins such as cresol-novolak-type epoxy resins, phenol-novolak-type epoxy resins, bisphenol-A-type epoxy resins, biphenyl epoxy resins, dicyclopentadien epoxy resins, and naphthalene epoxy resins. The epoxy resins may include silica as a filler. Examples of thermoplastic resins include polyphenylenesulphide, polycarbonate, polyethersulfone, thermotropic-liquid-crystal polymers, etc.
A metallic pipe made of an alloy of iron-nickel or an alloy of iron-nickel-cobalt is formed by drawing or electro-casting. The metallic pipe is used to produce the ferrule for an optical connector according to the present invention. The metallic pipe may be arranged along the entire length of the through-hole of the ferrule body or may be arranged only in the small-diameter portion of the through-hole. The metallic pipe preferably includes a tapered portion expanding toward the large-diameter portion of the through-hole. Advantageously, an optical fiber may be inserted into the through-hole and smoothly guided from the large-diameter portion toward the small-diameter portion.
A method for manufacturing a ferrule for an optical connector according to the present invention comprises the steps of inserting one end of a metallic pipe into a pin of a metallic mold to fix it thereto, arranging metallic molds around the metallic pipe to form a space therein, charging a resin for a ferrule body into the space, and curing the resin.
In another method for manufacturing a ferrule for an optical connector according to the present invention, a metallic pipe having a tapered portion expanding outwardly in the other end of the metallic pipe is used, the tapered portion is fixed by a movable body, and a resin for a ferrule body is charged and cured. The diameter of the movable body preferably corresponds with that of the large-diameter portion of the through-hole of the ferrule body.
In accordance with the ferrule for an optical connector of the present invention, when the material of the ferrule body is a resin and expands or contracts with changes in ambient temperature, since the metallic pipe (made of an alloy of iron-nickel or an alloy of iron-nickel-cobalt) has a smaller thermal coefficient of expansion than that of the resin, slippage of the butting position of optical fibers is reduced and reduction of optical transmission loss is achieved. When the metallic pipe is disposed at the inner wall of the through-hole, the ferrule is reinforced in its mechanical strength and breakage is prevented.
In accordance with a method for manufacturing a ferrule for an optical connector of the present invention, by fixing one end of a metallic pipe to a pin of a metallic mold and charging a resin for a ferrule body into a space formed around the metallic pipe, the accuracy in the position of the metallic pipe relative to the ferrule body is improved, resulting in better connection of optical fibers together.